The technical field of this invention is microelectrodes and, in particular, methods and devices which employ iridium microdisks capable of determining Cu2+ or Hg2+ using square wave anodic stripping voltammetry.
Determination at ppb levels of Cu2+ and Hg2+ is of special concern in both environmental and process monitoring. Copper is an essential element to the human diet, but intake of large quantities can be toxic. Soluble copper compounds in drinking water pose the greatest threat to humans. The average concentration of copper in tap water ranges from 20 to 75 ppb although many drinking water sources are higher due to the use of copper pipes and brass faucets. Being able to monitor and control the concentration of Cu2+ is also becoming important in the microelectronics fabrication industry, especially in very corrosive or harsh chemical conditions. On the other hand, mercury, even at low concentrations is one of the most toxic metals in the environment. Thus, the development of simple, reliable, and low-cost techniques for the determination of these metals is essential.
A variety of electrode substrates have been used for the determination of Cu2+. These have included; a mercury plated iridium microelectrode array (Jr-UMEA),1-4 a thin film gold electrode,5 a carbon disc microelectrode,6 a hanging mercury drop electrode,7 a mercury film electrode,8 and a mercury plated platinum microelectrode array.9 On-site analysis of copper has been performed using a mercury film glassy carbon electrode.10 In situ analysis of copper has also been performed with a mercury drop electrode,11 a mercury plated Ir-UMEA,1 and an agarose-coated mercury plated Ir-UMEA.3 The determination of Hg2+ has been achieved mainly with two types of electrodes, modified electrodes 12-18 and solid electrodes, such as carbon 19,20 and gold. 19,21-24 Determination of copper with simultaneous deposition of mercury has been performed using chemically modified 25, 26 and gold electrodes. 27-29 
For anodic stripping voltammetry (ASV), mercury is still widely used because of its large hydrogen overpotential, uniform surface, and suppression of underpotential peaks. However in many cases, such as measurements in environmental samples, in-vivo, in corrosive solutions, or where mercury plating is not feasible, mercury coated electrodes are practical. On the other hand, the direct use of solid substrates such as glassy carbon, Au, and Pt, has always been problematic because the stripping voltammograms display multiple peaks and distorted signals, making analysis difficult. The microfabricated iridium ultramicroelectrode (Ir-UME), coated with a mercury film, has been widely used for stripping voltammetric analysis of Cu2+ and several other heavy metals. 1-4 Even though bare iridium has a small potential window between hydrogen evolution and iridium oxidation, metals whose redox potential falls between these limits, such as Cu and Hg, may lend themselves to such an analysis.
The present invention is based, at least in part, on a discovery involving characterization and separate electrochemical determinations of Cu2+ and Hg2+ directly on a microlithographically fabricated array of iridium ultramicroelectrodes (Ir-UMEA), Square wave anodic stripping voltammetry was used to optimize experimental parameters such as supporting electrolyte, square wave frequency, and deposition time and potential. Reproducible stripping peaks were obtained for solutions containing low parts-per-billion (ppb) concentrations of either metal. It was discovered that excellent linearity was obtained for Cu2+ in the 20-100 ppb range and for Hg2+ in the 1-10 ppb range when using the bare iridium substrate. Detection limits were calculated to be 1 ppb (0.1 M KNO3 and 0.1 M HClO4, deposition time 180 s) and 5 ppb (0.1 M H2SO4, deposition time 120 s) for Cu2+ (S/N=3) and 85 ppt for Hg2+ (deposition time 600s). The experimental detection limits were determined to be 5 ppb for Cu2+ (deposition time 180 s) and 100 ppt for Hg2+ (deposition time 600s). Interference studies were performed and it was determined that Pb, Zn, and Cd had little or no influence on the copper signal. Tap water and spring water samples were analyzed for copper and good agreement was obtained with conventional methods. An unexplained effect of chloride ions on the iridium surface was noted. Further investigation by atomic force microscopy determined that changes on the surface occurred but could be eliminated when chloride leakage from the reference electrode is minimized. The solid state construction of the Ir-UMEA makes it a prime candidate for use in determining of Cu(II) and Hg(II) in chemically harsh environments.
In one aspect of the invention, an ultramicroelectrode includes: a) a metallic substrate; b) a first metallic oxide insulating layer; c) a first metallic adhesion layer; d) an iridium layer; e) a metallic bond pad layer; f) a second metallic adhesion layer; and g) a second metallic oxide insulating layer. The metallic substrate can be carbon, silicon, aluminum, phosphorous, gallium, indium, tin, antimony, selenium or germanium based., preferably silicon based. The first metallic oxide insulating layer can be a Group IV metallic oxide such as a Si, Ge, or Sn oxide, preferably a Si oxide. Typically the first metallic adhesion layer is a transition metal such as Ti, V, Cr, Sc, Nb, Mo, W, or, preferably Ti. The ultramicroelectrode also includes a metallic bond pad layer which can be Ni, Zn, Pd, Ag, Cd, Pt, Ga, In, or Au, preferably Au. The second metallic adhesion layer can be a transition metal such as Ti, V, Cr, Sc, Nb, Mo, W, or Ta, preferably Ti. The second metallic oxide insulating layer can be a Group IV metallic oxide such as Si, Ge, or Sn, preferably a silicone oxide. In preferred embodiment, the ultramicroelectrode of the invention does not include a mercury layer.
In certain aspects of the invention, one or more of the layers can be omitted from the ultramicroelectrode. The layers which can be eliminated can be determined by those skilled in the art and can include the omission of the first metallic oxide insulating layer and/or a second metallic adhesion layer and/or a second metallic oxide insulating layer; a first metallic adhesion layer and/or a second metallic adhesion layer and/or a second metallic oxide insulating layer; and a second metallic adhesion layer and/or a second metallic oxide insulating layer.
In a preferred embodiment, the ultramicroelectrode of the invention includes a) a silicon substrate; b) a silicon oxide insulating layer; c) a titanium adhesion layer; d) an iridium layer; e) a gold bond pad layer; f) a titanium adhesion layer; and g) a silicon dioxide insulating layer. In a preferred embodiment, the f) and g) layers have been partially or totally removed.
In still another embodiment, the present invention includes methods for producing an ultramicroelectrode by thermally growing a first metallic oxide insulating layer onto a metallic substrate followed by evaporating a first metallic adhesion layer onto the first metallic oxide layer. An iridium layer is the deposited onto the first metallic layer and a metallic bond pad layer is deposited onto the iridium layer. A second metallic adhesion layer is evaporated onto the metallic bon d pad layer and a second metallic oxide insulating layer is then deposited onto the second metallic adhesion layer. Preferentially, a portion or all of the second metallic adhesion layer and second metallic oxide insulating layer is removed.